This invention relates to a thermal flow sensor for detecting a flow rate with a heat-sensitive resistor.
A type of flow sensor in which flow rate is detected from the thermal balance condition of a bridge including a heat-sensitive resistor, has been disclosed, for instance, by Japanese Patent Application (OPI) No's 144216/1988 and 128125/1989 (the term "OPI" as used herein means an "unexamined published application"). In the flow sensor, in order to provide an excellent response characteristic, the heat-sensitive resistor is made of a metal wire or film which is considerably thin. Hence, it may be broken when a great force is externally applied to it, for instance, with a finger. In order to overcome this difficulty, nets are provided at both upstream and downstream of the flow sensor.
The conventional flow sensor is as shown in FIG. 4. In FIG. 4, reference numeral 1 designates the flow sensor; 2, a cylindrical housing through which fluid flows in the direction of the arrow A; 3 and 4, nets provided at both end openings of the housing 2 (hereinafter referred to as "an upstream end opening 3" and "a downstream end opening 4", respectively, when applicable); 5, a straight-pipe shaped detecting tube provided inside the housing; 6, a temperature sensor arranged inside the detecting tube 5, for detecting the temperature of fluid; 7, a heat-sensitive resistor set in the detecting tube 5 substantially at the middle; and 8, a control section incorporating a control circuit which includes a bridge circuit made up of the temperature sensor 6, the heat-sensitive resistor 7 and two resistors (not shown).
In the flow sensor thus constructed, the nets 3 and 4 are provided upstream and downstream of the heat-sensitive resistor 7 (hereinafter referred to as "an upstream net 3" and "a downstream net 4", respectively, when applicable) as was described above, and therefore, the heat sensitive resistor 7 cannot be touched directly from outside. Hence, this flow sensor is advantageous in that the heat-sensitive resistor 7 is scarcely damaged by external force. However, the flow sensor is disadvantageous in the following manner: Each of the nets 3 and 4 resists the flow of fluid, thus providing a pressure loss. Since the flow sensor has two nets 3 and 4, the loss of pressure is doubled, and therefore, it cannot be disregarded. One of the objects of the upstream net 3 is to regulate the flow of fluid. Therefore, the upstream net 3 may be beneficial although it causes a loss of pressure. On the other hand, the downstream net 4 is not so useful in regulating the flow of fluid, and is not very economical.